Cancer gene discovery and immunosurveillance studies using Sleeping Beauty mouse models

Cancer is the second leading cause of death in the United States. The majority of cases are caused by sporadic somatic mutation, which leads to cellular transformation over time. Therefore, cancer gene identification is a major focus of current research efforts. Understanding how key driver mutations result in cancer could lead to the design of better targeted therapies.

The Sleeping Beauty (SB) transposon system can be used to identify driver mutations in a variety of tumor types. SB mutagenesis mimics the sporadic accumulation of somatic mutations found in spontaneous human cancers. This system also has an additional benefit over chemical carcinogenesis models in that key cancer gene candidates are easily identified with high-throughput sequencing and subsequent bioinformatic analysis. Using SB, our lab recently identified a novel oncogene involved in non-melanoma skin cancer called Zmiz1, the biological function of which is not well studied.

A major focus of my thesis work was to characterize Zmiz1 and its role in skin cancer. This gene encodes a protein with predicted E3 SUMO ligase activity. My work has provided the first evidence firmly establishing an oncogenic role for Zmiz1 in cutaneous malignancy, thereby generating a novel transgenic mouse model of skin carcinogenesis. Importantly, we observed tumor-specific overexpression of an endogenous ZMIZ1 isoform in human squamous cell carcinomas.

Non-melanoma skin cancer is the most common malignancy worldwide, and it disproportionally affects immunosuppressed patients. One proposed explanation for this is the concept of tumor immunosurveillance, whereby the immune system suppresses tumor growth. When the immune system is compromised, transformed cells can develop into tumors. However, immunocompetent people also develop cancer, despite an intact immune system. It is thought that while the immune system is keeping transformed cells from forming a tumor, it simultaneously influences the acquisition of new mutations that eventually allow escape from immune detection and clearance. This process, called immunoediting, is widely believed to be dependent upon the adaptive immune system and another focus of my research was studying immunoediting mechanisms using SB mutagenesis.

Subtle differences were observed in the SB-induced mutation spectra of tumors generated in immunocompetent mice and immunocompromised mice, suggesting that some level of lymphocyte-dependent immunoediting of tumors had occurred. However, the adaptive immune system was not effective in suppressing tumor formation, which is in contrast with previously published data. My work represents an independent and original assessment of the immunoediting process, and cautions against reliance on a single animal model to study this area of cancer biology.